1. Field of the Invention
The present invention is directed to an apparatus for identifying the spatial distribution of electrical impedance within an examination region of a living subject, as well as to a method for identifying the spatial distribution of electrical impedance within an examination region of a subject.
2. Description of the Prior Art
Various techniques and systems are known for identifying the spatial distribution of electrical impedance within an examination region of a living subject. Systems are known which have the following features.
A source of electrical current is connected via electrical connecting lines to at least two feed electrodes, the feed electrodes being suitable for impressing a feed current from a current source in an examination region of a subject. The impressed current causes a current distribution within the examination region, corresponding to the distribution of electrical impedance and to the position of the electrodes. A magnetic field measuring instrument is then used to acquiring a spatial distribution of characteristic quantities of the magnetic field which arises due to the aforementioned current distribution. This magnetic field is measured at measuring points outside the examination region. The output of the magnetic field measuring instrument is supplied to an evaluation unit, which reconstructs an equivalent distribution of the current density within the examination region based on the spatial distribution of the characteristic quantities. The equivalent current density distribution at the measuring points is that which would be generated by a theoretical magnetic field which best coincides with the measured magnetic field caused by the distribution of the current.
It is known to identify the electrical impedance within a subject non-invasively by means of so-called electrical impedance tomography as described, for example, in the article "Electrical Impedance Tomography and Biomagnetism," Webster, published in Book of Abstracts, 8th International Conference on Biomagnetism, Muenster, Aug. 18-24, 1991, or in the article "Electrical Impedance Tomography; the Construction and Application to Physiological Measurement of Electrical Impedance Images," Brown et al., published in Medical Progress Through Technology 13, pp. 69-75, 1987 Yartinus Nijhoff Publishers, Boston.
In this known techniques, alternating currents having frequencies in the range from 10 through 50 kHz are impressed on the body via electrodes applied to the subject. Tomograms of the conductivity distribution or impedance distribution are calculated in a tomographic reconstruction from the differences in potential which thereby arise between the electrodes.
Published electrical impedance values available in the literature, which were acquired ex vivo or in the course of one-time in vivo examinations, preferably on an animal model, are usually used for producing body-adapted models. The relative magnitude in specific body regions as well as the chronological variation of electrical impedance can be directly consulted for making a medical diagnosis. For example, in vessel or tumor diagnostics as well as in combination with the administration of medications or other therapy measures, deviations of the electrical impedance from standard values, or standard distributions, can be evaluated.
It is proposed in the article "Magnetic Imaging of Conductivity," Ahlfors et al., Proceedings of the Annual International Conference of the IEEE Engineering in Medicine and Biology Society, Vol. 14, Paris, France, Oct. 29-Nov. 1, 1992 Part 5, pp. 1717-1718, that a current be impressed on a subject via surface electrodes for identifying the conductivity distribution of the subject, and that the magnetic field generated thereby then be evaluated. It is assumed in this article that only small deviations from a prescribed conductivity distribution occur in the examination region. It is assumed in a further approximation that the magnetic field varies in the same way, as though an additional, equivalent current density distribution were present in the examination region. This additional, equivalent current density distribution is identified by means of a minimum norm estimate from the inverse solution of the measured magnetic field distribution with localization methods which are employed in the field of biomagnetism. The conductivity changes are identified from the equivalent current density distribution, by dividing the equivalent current density distribution by the electrical field of prescribed conductivity distribution, without taking the deviations into consideration.
It has now been shown that the measured values, heretofore thought to arise primarily due to the characteristics of the subject, are in fact dominated by the magnetic field of the entire conductor loop which includes the connecting feed lines, so that a non-uniform current density in the examination region can only be acquired as weak measured values. The magnetic field arising from the connecting lines could be compensated, for example, by subtracting a calculated or measured value corresponding to the contribution of the connecting lines. This would result, however, in a very low sensitivity of the apparatus.